Where miles of cable are involved, how do people determine where the fault lies and decide where to dig for initiating repairs? The method involves something very similar to how people determine the depth of a well, the distance of a cliff or the location of a thundercloud – by echolocation. If you know the speed of sound in air, you can find the distance of the sound source from the product of the amount of time sound is taking to travel the distance to its speed in air. For example, light travels much faster than sound; therefore, light from a thunderclap precedes its sound. By timing the gap between seeing the flash of light and hearing the thunder, it is easy to tell how far away the thunderclap occurred.
When an electrical pulse is directed into one end of the cable, it travels down the length until it meets a change in the cable’s impedance. This may be a fault in the cable or it may simply be its other open end. Whatever the situation, the change in impedance causes the pulse to turn back to its point of origin. The time gap between the original pulse and its return represents the length it has traveled. Therefore, if it has returned in, say 30ns, instead of the 100ns expected, the fault is at about 1/3rd the cable’s length from the end where the pulse was injected.
Engineers usually rig up an oscilloscope and a pulse generator for the purpose. Knowing the cable’s characteristics is necessary to set the pulse generator’s output impedance. The pulse generator needs to output a narrow pulse of about one to 100ns, with as small a duty cycle as possible – the two parameters depending on the length of the cable under test. The pulse voltage is not critical – 1V peak is enough.
The oscilloscope’s trigger level should be just under the peak voltage of the 1V pulse. The time base should be set just long enough to display one pair of the 1V pulses generated. That completes the setup.
As you launch the pulse into the cable, it triggers the oscilloscope sweep. The pulse now continues to the other end of the cable, until it encounters an open end. Since energy cannot be destroyed, the pulse is reflected back to the generator. When it passes the oscilloscope, it is displayed again. You can differentiate the reflected pulse from the original by the reduction of its amplitude and a difference in the rise/fall slopes. This happens because of attenuation when traveling within the cable and a loss of high-frequency harmonics. Although there may also be additional reflections caused by input capacitance of the oscilloscope, the echo of interest is only the first one after the original pulse was launched.
The round trip time is dependent on the cable length, which is usually known. For most cables, the pulse will travel at about 66% of the speed of light in vacuum (300m/µs). That makes its speed within the cable about 200m/µs. You may have to play with the time base and the pulse period until you can see both the launched and the reflected pulse.
